Southern California's Woolsey and Hill Fires: An Analysis of Fire Weather Conditions and High-Resolution Model Output to Improve Weather Forecasts and Decision Support Services

Santa Ana winds are dry and oftentimes hot east and northeast winds that blow from the deserts east of the Sierra Nevada to the coast of southern California. When combined with critically dry vegetation, these winds are a major cause of large wildfires, which can be very destructive given the vast residential areas that sit within the wildland urban interface in southern California. Forecasting these events can present significant challenges due to small scale atmospheric processes that are enhanced by the area’s complex coastal terrain. In providing decision support services for its core fire agency partners making costly decisions for pre-positioning and augmenting resources, it is critical National Weather Service forecasters have access to numerical model guidance that can support the most accurate and timely forecast possible. This presentation will demonstrate the utility of such guidance in properly anticipating one such Santa Ana event that led to two large wildfires, possible explanations for why certain mesoscale model configurations can correctly forecast the complex conditions that led to the fires, and how that guidance can be used to give partner fire agencies support for fast and efficient response.

A challenging period was ahead for NWS forecasters situated in Oxnard, California in early November 2018 as a strong Santa Ana wind event was forecast to hit the area. Very little rainfall occurred in the months leading up to this event contributed to very dry vegetation across the foothill and mountain areas of southern California. Dry fuels combined with an extremely dry air mass and strong Santa Ana winds brought the fire danger parameters to near 30-year highs. The Woolsey and Hill fires erupted during the afternoon hours on November 8, 2018 in an area just northwest of Los Angeles in the Santa Monica Mountains. The most destructive fire of the two, the Woolsey fire burned over 96,000 acres, destroyed 1,643 structures and damaged 364 others for a total of $6 billion in property loss. The fire also shut down two major highways near the Ventura-Los Angeles County border, hampering evacuation efforts and emergency response. The study presented will focus on the weather conditions leading up to and during the Woolsey and Hill fire starts and comparing surface wind and relative humidity observations to different mesoscale model simulations using different resolutions and parameterizations. The results will help show why certain parameterizations and model configurations are able to resolve the details of these fire weather events in southern California - configurations which can ultimately help in shaping the operational models used by the NWS to provide accurate and timely decision support services to its partners needed to protect their communities.